Thank you, @spikey582 for this wonderful feedback on the story. First of all yes, I think that Robbie definitely feels like an outsider in the family, despite Nicola's recent 'U' turn and undoubted efforts from Sue to make him feel a part of the family. It does take a while for this to happen and it can't be forced. Robbie hasn't helped by putting up his own mental barriers but eventually, even these will be broken down. Robbie has used the situation to his own advantage causing friction with Nicola and we know now, Daniel. The way that he has used people isn't too dissimilar to what Alex has done. A watered down version perhaps.
Clearly, Don has serious issues and we already know that he is homophobic, but Robbie convinced him in the past that he was straight and Don still believes this. It doesn't excuse his deplorable reaction, but he sees their relationship as purely sexual and in his mind 'deviant'. In actual fact, their relationship has been quite tame, with very little sex.
It would be interesting of course to see what happened after Robbie left and what has been said but the story is told through Robbie's eyes and we only know what he knows. We will find out more soon though. Thanks again spikey.